1. Field of the Invention
The present invention relates to a heating device and a fixing device, both for use in an image forming apparatus; a method of controlling temperature of a heating member; and an image forming apparatus.
2. Description of the Related Art
Copy machines, printers, and facsimile machines form images on recording media such as plain paper and OHP sheets. There are various systems for forming images on recording media. An electrophotographic system is one of those systems and is widely used due to high-speed performance, high image quality and low cost. An image forming apparatus, such as a printer and a facsimile machine, using the electrophotographic system includes a transfer unit. The transfer unit forms a latent image according to image information such as electronic information or optical information, develops the latent image with toner (image developing agent) made of thermoplastic resin containing pigment, and transfers the developed image onto a recording material by using a direct or indirect (transfer) system to form a toner image thereon. A fixing device is used to permanently fix the toner image transferred on the recording material by heat. A heating roller system is currently most widely used in such a fixing device because of its high speed and safety performance.
The heating roller system includes a heating roller (also called as a fixing roller) heated by the heat source and a pressure roller opposing the heating roller, together forming a nip portion therebetween. A sheet of a recording material is passed through the nip portion so that toner on the sheet is fixed thereon by heat. A typical heating roller system uses a halogen lamp as the heat source. The halogen lamp is disposed inside the fixing roller so as to heat the fixing roller from its inside to increase the surface temperature of the fixing roller to an appropriate temperature. Problems with such a heating roller system using the halogen lamp are that reduction of the heat capacity (thickness) of the fixing roller is limited and that the start-up is slow due to slow start up of the halogen heater.
To solve these problems, a belt heating system has been developed. The belt heating system uses an endless sheet-like belt as a heating belt in place of the heating roller. The heating belt and a pressure roller form a pressure-contact portion (a nip portion) therebetween. A sheet of recording material is passed through the nip portion so that an unfixed toner image on the recording material is fixed thereon by heat. The heating belt moves over a heating body (usually serving also as support rollers). The heating belt is heated by the heating body so as to heat and fix the toner image on the recording material. A heating device using the belt heating system can use a ceramic heater or the like having a low heat capacity as a heating body, and can use a thin heat-resistant sheet having a low heat capacity as a belt member of the heating belt. Therefore, compared with a heating device of a heating roller system that uses a heating roller having a high heat capacity, the heating device of the belt heating system uses less power and achieves shorter waiting time, and thus can provide advantages such as quick starting (see Japanese Patent Laid-Open Publication No. H04-44075 (Patent Document 1)).
However, in the sheet-like heating belt having the reduced heat capacity, the heat flow in the width direction of the heating belt (the direction perpendicular to the belt moving direction, i.e., the longitudinal direction of the nip portion) is blocked. Accordingly, when a small size recording sheet is passed over in contact with only a part of the heating belt in the width direction of the heating belt, a non-sheet-passing portion of the heating belt is overheated, resulting in reducing the service lives of the heating belt and the pressure roller. One way to solve this problem is to increase the interval of feeding the recording sheets when feeding small size recording sheets and thus lower the throughput of passing the sheets, thereby allowing heat transfer in the heating belt and providing cooling time. However, providing time for the heating belt to reach uniform temperature significantly lowers the image forming speed of the image forming apparatus. This problem applies more or less to the above-described heating roller system as well.
In recent years, use of an electromagnetic induction heating system has been studied as a way of heating the fixing roller. This system includes a magnetic flux generating unit that generates an alternating magnetic flux, which produces an eddy current to cause electromagnetic induction heating of a fixing roller having a conductive layer. This electromagnetic induction heating system can directly heat the target, the surface layer of the fixing roller, and therefore can heat the fixing roller more quickly compared to the halogen heater and can reduce the waiting time for starting operations. Further, the speed of supplying heat is high enough to enable high-speed operation of the image forming apparatus.
Japanese Patent Laid-Open Publication No. 2000-214702 (Patent Document 2) discloses a fixing roller of an electromagnetic induction heating system. The fixing roller includes five layers, a support layer (core layer), a sponge layer (foamed layer), an electromagnetic induction heat generating layer, an elastic layer, and a releasing layer in this order from inside to outside. The heat generated by the heat generating layer is blocked by the sponge layer, so that the elastic layer and the releasing layer at the surface of the fixing roller can be quickly heated. With this configuration, the surface of the fixing layer is quickly heated to a required temperature and, after heat is transferred to a recording medium such as paper, the fixing roller is quickly reheated. This permits higher speed operation than that using a halogen lamp.
A problem with the electromagnetic induction heating system is that, because the electromagnetic induction heat generating layer is thin, it is difficult to control the temperature distribution in the longitudinal direction of the fixing roller as in the case of the belt heating system. In some fixing devices, when continuously fixing images on small size media, a part of or the entire fixing roller is overheated. A typical image forming apparatus is capable of forming images on several types of recording media of different widths. The term “recording media of different widths” indicates various standard size recording media of JIS A and B sizes and non-standard size recording media. Even in the case of recording media having the same size (e.g. A4 size), if one is fed in the portrait orientation and the other in the landscape direction, they are handled as recording media of different widths. When a fixing device fixes images on recording media of different widths, the heat distribution in the fixing member in the width direction varies due to the different widths of the recording media, resulting in a temperature variation. For example, in the case of fixing an image on a small width recording medium, a region (a sheet-passing-region) corresponding to the width of the recording medium loses more heat and has lower fixing temperature than a region (non-sheet-passing region) on which the recording medium does not pass. This phenomenon becomes especially pronounced when small width recording media are continuously passed over.
If the fixing temperature of the fixing roller across the entire width thereof is controlled based on the fixing temperature of the horizontal center portion of the fixing roller as a reference temperature, which center portion is always in the sheet-passing-region, although the fixing temperature of the horizontal center portion of the fixing roller can be maintained constant, the fixing temperatures of the opposite horizontal end portions of the fixing roller are (excessively) increased. If a large-width recording medium goes through a fixing process using the fixing roller whose opposite lateral end portions have increased fixing temperatures, hot offset is produced in portions of the recording medium corresponding to the portions of the fixing roller having increased temperatures. Moreover, if the fixing temperatures of the opposite lateral end portions exceed the allowable temperature limit of the fixing roller, the fixing roller can be damaged due to heat. On the other hand, if the fixing temperature of the fixing roller across the entire width thereof is controlled based on the fixing temperatures of the opposite horizontal end portions of the fixing roller as a reference temperature, although the fixing temperatures of the opposite horizontal end portions of the fixing roller are controlled to the desired temperature, the fixing temperature of the horizontal center portion of the fixing roller decreases. If a recording medium goes through a fixing process using the fixing roller whose lateral center portion has a reduced fixing temperature, cold offset is produced in the portion of the recording medium corresponding to the portion of the fixing roller having the reduced temperature.
To solve these problems, a halogen heater type fixing device uses plural heaters as the heat source. The heaters are disposed to emit lights on the center portion and end portions of the fixing roller and are individually controlled so as to control the temperature of the fixing roller. However, in the case of the electromagnetic induction heating system that heats a target by a magnetic flux generated by a coil, providing separate coils for heating the center portion and the end portions as in the case of the halogen heaters is not a practical solution because many problems arise such as cost increase and interference between the coils.
Another solution may be to provide, in addition to an exciting coil for electromagnetic induction heating, a secondary demagnetizing coil in a region corresponding to a non-sheet-passing region. The secondary demagnetizing coil generates an inductive motive force and an inductive current due to fluctuation of magnetic flux of the exciting coil, so that the inductive motive force and the inductive current reduce the magnetic flux in the non-sheet-passing region, thereby preventing overheating. When reducing heat generation, the secondary demagnetizing coil is closed by a switching circuit, such as a relay, a FET, or an IGBT, so as to generate a current. When not reducing heat generation, the secondary demagnetizing coil is opened so as not to activate the secondary demagnetizing coil, thereby preventing generation of a demagnetizing magnetic flux. Heat generation is thus controlled by opening and closing the switch.
For instance, Japanese Patent Laid-Open Publication No. 2001-60490 (Patent Document 3) and Japanese Patent Laid-Open Publication No. 2001-135470 (Patent Document 4) disclose heating rollers as described below. A heating roller includes therein a magnetic core comprising three pieces and extending in the width direction of the sheet; an exciting coil disposed around the magnetic core and wound to form a layer on the inner surface of the heating roller; and demagnetizing coils (cancel coils) wound around the outer pieces of the magnetic core and extending in the direction perpendicular to the layer of the exciting coil. When fixing an image on a sheet of recording material of the maximum width, the demagnetizing coils are opened by a switching circuit so as not to be activated. Therefore, the image is appropriately fixed across the entire width of the sheet of the maximum width. When fixing an image on a smaller width sheet, the demagnetizing coils are closed by the switching circuit. Accordingly, at the end portions of the heating roller in the sheet width direction, not only an inductive current (eddy current) due to fluctuation of the magnetic flux of the exciting coil, but also a back electromotive force (and a current induced by the force) are generated. Thus, temperature rise is reduced at the end portions of the heating roller.
Japanese Patent Laid-Open Publication No. 2005-108603 (Patent Document 5) discloses a fixing device that has a different coil arrangement from that of the above-described fixing device. In the fixing device of Patent Document 5, a demagnetizing coil is disposed along the layer of an exciting coil. With this arrangement, the demagnetizing coil can effectively cancel the magnetic flux of the exciting coil, and thus demonstrate the increased effect of reducing temperature rise.
As described above, the electromagnetic induction heating system, which has many advantages including reduced power consumption and quick start, can deal with a variation of widths of recording sheets to some extent. However, because the temperature control using the secondary demagnetizing coil as described above relies on the On/Off control of the secondary demagnetizing coil (hereinafter referred to also as a demagnetizing coil), it is difficult to provide precise temperature control. For example, in the case of the fixing rollers disclosed in Patent Documents 3 and 4, because the greater part of each demagnetizing coil, which extends in the direction perpendicular to the layer of the exciting coil, excluding an end portion of the demagnetizing coil facing the exciting coil is spaced apart from the exciting coil, leakage magnetic flux (magnetic flux of the exciting coil not passing through the magnetic core) does not pass through the demagnetizing coil. Therefore, the demagnetizing coil has less effect of reducing temperature rise, resulting in an insufficient temperature reduction of the heating roller. In the case of the fixing device disclosed in Patent Document 5, because the demagnetizing coil is disposed to face a heating roller with the exciting coil therebetween, a leakage magnetic flux (magnetic flux of the exciting coil not passing through a magnetic core (a holder)) does not pass through the demagnetizing coil. Therefore, the demagnetizing coil has less effect of reducing temperature rise, resulting in an insufficient temperature reduction of the heating roller.
As mentioned above, since there is a gap between the exciting coil and the demagnetizing coil due to the arrangement thereof, leakage of magnetic flux is inevitable. To enhance the demagnetizing effect, the number of turns of the demagnetizing coil may be increased. However, increasing the number of turns of the demagnetizing coil increases the entire size of the heating device. If the magnetic core is disposed on the path of the exciting coil and the demagnetizing coil for increasing their connection, or if the size of the demagnetizing coil is increased, the current applied to the demagnetizing coil may become too high depending on the condition of supplying power to the exciting coil. If the current value of the demagnetizing coil becomes excessively high, the current may exceed the allowable current of a switching element that controls opening and closing of the circuit. Further, the temperature of the demagnetizing coil may exceed the allowable temperature limit of the wires thereof. If a high current is unexpectedly applied to the demagnetizing coil, the effect of reducing heat generation may be excessively increased, so that the temperature of the non-sheet-passing portion may be excessively reduced.